Microwave heating device

ABSTRACT

A microwave heating device for heating load(s) and including a cylinder-shaped cavity enclosed by a periphery wall, the cavity is provided with a microwave feeding device. The heating device may have a dielectric wall structure arranged inside the cavity between the periphery wall and the load(s). The microwave feeding device may be arranged to generate a microwave field being an arch surface hybrid mode having TE and TM type properties inside the cavity to heat the load(s).

FIELD OF THE INVENTION

The present invention relates to a microwave heating device, a microwaveheating system and a method according to the preambles of theindependent claims.

BACKGROUND OF THE INVENTION

Cavities and applicators for microwave heating of materials aretypically resonant in operation, since such a condition results inpossibilities of achieving a high microwave efficiency. Typicalcavity/applicator loads have either a high permittivity such as 10 to 80for polar liquids and compact food substances, or a lower permittivitybut then also a low loss factor and a larger volume, such as in dryingoperations. In both these cases there is a need for the microwave energyto be reflected and retro-reflected many times in the cavity/applicatorin order for a sufficient heating efficiency to be obtained. However,resonant conditions entails a limitation of the frequency bandwidth ofproper function.

There are three methods in use to overcome the practical problem oflimited resonance frequency bandwidth:

-   -   Use of multiple resonances in a comparatively large cavity. At        least one resonance will then exist at the operating frequency        of the generator such as a magnetron. This type of cavity is        easy to use but has the drawback of variable and quite        unpredictable heating patterns and microwave efficiency for even        slightly different loads, particularly if these are small.    -   Use of some adjustment means for the resonant frequency in a        single mode cavity/applicator. Mechanical means such as movable        shorting plungers are cumbersome and require good galvanic        contact. A more practical but still mechanically operated device        is a non-contacting deflector described in WO-01/62379.    -   Use of adjustable frequency generators.—Low power semiconductor        generators or expensive TWT tubes may be useful, but another        problem then occurs: that of the limits of the established ISM        bands. For operating frequencies outside these, complicated        shielding and filtering is needed.

If the required frequency variations are within for example the allowed2400 to 2500 MHz, systems of the third kind above intended for a limitedrange of load geometries or permittivities may work well. The reducedresonance frequency span in use must then be inherently designed intothe microwave applicator.

It may also be possible to achieve negative feedback of the applicatorplus load resonant frequency by utilising a combination of applicatorcavity and internal load resonant properties. Such systems are thenlimited to particular and rather narrow load geometries and dielectricproperties, such as disclosed in U.S. Pat. No. 5,834,744.

SUMMARY OF THE INVENTION

An overall object of the present invention is to achieve a microwaveheating device having a stable resonant frequency for a large variety ofload geometries and permittivities, and also being less complex, morerobust and less expensive than prior art arrangements.

This object is achieved by the present invention according to theindependent claims.

Preferred embodiments are set forth in the dependent claims.

The present invention relates to a microwave enclosure which may be apartially open or closed resonant applicator incorporating a dielectricstructure between a periphery wall and the load. The applicator is inprinciple mathematically cylindrical, which means that it has a definedlongitudinal axis and a constant cross surface area (including that ofthe dielectric structure) along this axis. The type of mode in theapplicator is essentially fieldless along a longitudinal axis in acentral region of the applicator.

In typical single mode resonant applicators, the resonant frequency isreduced when a load is inserted, and if the load is not so large that itmodifies the applicator mode pattern significantly, a higher loadpermittivity further lowers the resonant frequency. The device accordingto the present invention is essentially self-regulating by the modebeing of a particular hybrid type. The mode can be said to consist of aTE part (with the axis as reference) and a TM part, the latter having an“inherent” higher resonant frequency and becoming stronger in relativeterms when a load is inserted into the applicator, so that acompensation of the lowering of resonant TM mode frequency occurs.

The hybrid mode is of the HE type and have all six E and H orthogonalfield components. It may exist in its basic form in a circularlycylindrical waveguide or cavity having a concentric dielectric at theperiphery or further inwards. A TE mode with higher first (rotational,m) index than zero has this theoretically known property. However, themode is to be fieldless at the longitudinal central axis in the presentcase, so the lowest first index is 2. Such applicators may be quitesmall, but applicators with first indices over 10 are also possible,resulting in a very wide application area for loads a fraction of a mLup to tens of L in volume, at 2450 MHz. An applicator for small loadsmay be basically closed and sector-shaped with a minimum sector angle of360 m/4; in such cases an integer index is no longer needed. Anapplicator for large loads that are for example tube-shaped may becircular and open in central areas at the axis, for load insertion.

SHORT DESCRIPTION OF THE APPENDED DRAWINGS

FIG. 1 schematically illustrates the TE₄₁ mode.

FIG. 2 illustrates a cross-sectional view of a microwave heating deviceaccording to a first preferred embodiment.

FIG. 3 illustrates a variant of the first embodiment in a perspectiveview.

FIG. 4 illustrates an alternate feeding means applicable for the presentinvention in a perspective view.

FIG. 5 shows a cross-sectional view of a the device shown in FIG. 4.

FIG. 6 shows in a perspective view a second preferred embodiment of thepresent invention.

FIG. 7 shows the second preferred embodiment in a cross-sectional view.

FIGS. 8 and 9 show cross-sectional views of variants of the secondpreferred embodiment.

FIG. 10 shows in a cross-sectional view 6 microwave heating devicesshown in FIG. 7 arranged together.

FIGS. 11 and 12 show cross-sectional views of different alternativeembodiments of the present invention.

FIG. 13 shows a cross-sectional view of a third preferred embodiment ofthe present invention.

FIGS. 14 and 15 illustrate cross-sectional views of two embodimentsaccording to the present invention of microwave heating devices providedwith large radial airspaces.

FIG. 16 shows a cross-sectional view of a fourth preferred embodiment ofthe present invention.

FIG. 17 shows a block diagram of a system for using the microwaveheating device according to the present invention.

Like numbers refer to like elements having the same or similar functionthroughout the description of the drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention deals with and depends on certain properties of archsurface modes. Such modes can exist in cylindrical cavities withcircular and elliptical cross sections, as well as with some polygonalcross sections. It has, however, been found that the deviations fromsmooth surfaces caused by the edges at corners may be unfavourable insome circumstances even with more than regular 12-sided polygonal crosssections.

Therefore, and since elliptical cross sections offer advantages only insome distinct cases, mainly circular cross sections—and in particularcross sections consisting of a circular sector—are dealt with here. Moredetailed extensions for non-circular peripheral geometries will followlater.

As a first illustrative example, the TE₄₁ mode is now dealt with (seeFIG. 1). It has 8 maxima of the axial magnetic field (which is thedominating magnetic field direction) along the circular periphery of anempty waveguide or cavity. In the figure, the magnetic field is dashedand the electric field (which only exists in the plane perpendicular tothe axis) is drawn as continuous lines.

An air filled empty TE₄₁₁ cavity is resonant at 2450 MHz when it has anaxial length of 100 mm and is about 260 mm in diameter. Most of theenergy is concentrated at the periphery, and can be described as twopropagating waves along that, in opposite directions then setting up astanding wave pattern.

Arch surface modes can exist in confining geometries having a curvedouter metal wall. In the simplest case, that of circularly cylindricalwaveguides and resonators, they are defined by the axis being fieldless.Hence, in the common system of circular mode notation, the first(circumferential variation, defined to be in the φ-direction) index is“high”, the second (radial variation, defined to be the ρ-direction)index is “low”, and the third, axial (defined to be the z direction)index is arbitrary.

The most common polarisation type for arch surface modes is TE, whichmeans that there is no z-directed E field. Typically, there is adominating z-directed magnetic field (and hence a φ-directed wallcurrent) at the curved metal surface. The first index must be at least2.

For TM modes, there is no z-directed H field, and typically a dominatingz-directed E field some distance away from the curved wall. The firstindex must also here be at least 2.

TE modes generally couple less efficiently to dielectric loads which arecharacterised by having a larger axial (circumferential, polygonal orcircularly cylindrical) surface than its “top” and “bottom” (constant zplane) surfaces, since their E field is only horizontally directed andwill therefore be perpendicular to any vertical load surface. They alsohave higher impedance than that of free space plane waves, which againresults in a poorer coupling to dielectric loads which are inherentlylow impedance. One may simplify the situation by saying that there is nofirst-order coupling mechanism by TEz modes to loads with dominatingz-directed dimensions. As a result, a higher quality factor (Q value) isneeded for good power transfer to the load, but this entails a morenarrow frequency bandwidth of the resonance needed for efficient heatingof in particular small loads.

TM modes have z-directed E fields and are low impedance. They thereforecouple significantly better to loads as above. However, that also meansthat loads which are not very small may influence the overall systemproperties, by for example causing a very significant resonant frequencychange which offsets the advantage of a lower Q value (and by that thelarger frequency bandwidth of the resonance).

A subgroup of the arch surface modes are the arch surface modes bound bya dielectric wall structure in the form of e.g. slabs, tiles or a planeor curved sheet.

The present invention is directed to this subgroup of arch surfacemodes, i.e. to microwave heating devices that include a closed cavityprovided with a dielectric wall structure essentially located between aperiphery wall of the cavity and one or many loads to be heated insidethe cavity.

In circular (and also elliptical) cylindrical geometries it is thenpossible to introduce diametrical metal sidewalls in the axialdirection, to create 8 independent cavities or waveguides. The smallestsuch sector-shaped cavity is 45° and is obtained with cut planes at 0°and 45° from e.g. the 6 o'clock direction in FIG. 1. The fieldproperties (resonant frequency, etc.) do not change in such cases.

Such a sector waveguide can be considered to have a mode which becomesevanescent towards the edge (at the former axis). Hence, a load locatedclose to this tip will be heated by some kind of evanescent coupling ofthe waveguide mode. It is then of great importance that the fieldimpedance of the radially inwards-going evanescent mode is high andinductive. Since the load is supposed to have a significantly largerpermittivity than air, the wave energy having reached the load is nolonger evanescent.

A significant absorption can take place, provided the wave energydensity has not fallen off “too much” at the load location. However aload located near the edge tip will couple very poorly.

Obviously, by locating a smaller load closer to the arched part of thecavity, the coupling will become stronger. It is also influenced by theload location in the angular direction, since the strength of inparticular the magnetic field varies with location relative to themicrowave feed or radial wall locations.

In the following is given an introduction of arch surface modedefinitions and polarisations.

Microwaves may propagate along the boundary between two dielectrics,provided one of the regions has some losses (a so-called Zennek wave).Waves may also propagate without losses, along and bound to a losslessdielectric slab (a so-called dielectric-slab waveguide). A variant ofthe latter is that the dielectric has a metal backing on one side—as isthe case for the present invention; the modes are then trapped surfacewaves.

The lossless propagation means that there is no radiation away from thesystem, in all the cases above—if there is no disturbing or absorbingobject in the vicinity of the surface.

In U.S. Pat. No. 3,848,106 is disclosed a device that uses surface wavesfor microwave heating. The mode type is of the TM type, with thepropagation in the direction (z) in the feeding TE₁₀ waveguide inessence having a dielectric slab filling being open to ambient in onebroadside (a side). Hence, the mode field just outside the dielectricfilling has no z-directed magnetic field but E fields in all directions.The mode used in the cavity according to the present invention is ahybrid mode that is defined herein as a mode where both E- and H-fieldsexist in the z-direction (being the longitudinal direction of thecavity.). In the hybrid mode the TE- and TM-modes exist and haveradially directed H-fields. As an example: The hybrid mode HE₃₁₁ has all6 components in a cavity provided with rotationally symmetricaldielectric structure.

Below is a theoretical reasoning regarding arch surface modes incircular waveguides and cavities.

As in any cylindrical empty metal tube of arbitrary cross section, therecan be two distinct classes of modes in a circular waveguide: TE and TMto z. That means that one of the six E and H components must be missing.That is z-directed E and H, respectively.

It is of major importance for the invention that TM arch surface modeswith the same three indices as TE modes have a higher resonant frequencyin the same cavity (i.e. known diameter and length).

As an example, for the TE₃/TM₃ modes, the x′/x quotient is 4.42/6.38, tobe inserted in the formula:

$f_{R} = {\frac{c_{0}}{2\;\pi\; a}\sqrt{x_{mn}^{\prime\; 2} + ( \frac{p\;\pi\; a}{h} )^{2}}}$where f_(R) is the resonant frequency, c₀ the speed of light, mnp themode indices, a the cavity radius and h its height.

It is also important that all TE and TM modes in circular waveguides areorthogonal (except for the TE₀ and TM₁ series, which are, however, notarch surface modes). Hence, they cannot couple energy to each other.

When a circular waveguide has a concentric dielectric filling(ring-shaped along the periphery or a distance away from it, or acentral rod), the modes no longer become TE or TM to any cylindricalco-ordinate, except for rotationally symmetric fields (arch surfacemodes are not that). This has been known since long as a theoreticalcuriosity.

With references to FIGS. 2 and 3 are given the basic designs andproperties of a first preferred embodiment of the present invention.

It is to be understood that when the direction of reference is changedfrom a longitudinal in a rectangular system to the cylindrical system,the rectangular TM₀ mode becomes similar to the circular TE_(m1) mode.Even if fully circular applicators are possible and may be feasible todesign and use, a reduced geometry may be preferred for the purpose ofheating of small loads. Not only is a smaller cavity obtained, butunwanted modes are also more easily avoided.

There are also other possible advantages by the particular current andfield intensity distributions on flat metal axial cavity walls at anangle, with a dielectric wall structure along the curved sectorperiphery.

Thus, two variants of this first embodiment are shown in FIGS. 2 and 3,respectively. FIG. 2 shows a cross-sectional view in the xy plane, of a120° sector applicator (or cavity) comprising a periphery wall 2, sidewalls 4, a load 6, a dielectric wall structure 8 and a microwave feedingmeans 10, where the dielectric wall structure comprises four flatdielectric tiles.

FIG. 3 illustrates in a perspective view a similar heating device buthere with a dielectric-coated periphery wall 2. For both FIGS. 2 and 3:The dielectric wall structure is about 7 mm thick and has a typicalpermittivity of about 7.5. The loads are quite large (30 to 40 mmdiameter) and the applicator radius is about 85 mm; the height is about80 mm and the operating frequency is in the 2450 MHz ISM band.

It should be noted that when sector-shaped cavities are used, there isno longer a requirement on specified sector angles for obtainingresonances. Therefore, there is now a continuum of angles versus radius.Since analytical formulas involving integer order Bessel functions canbe used for integer indices such as 3 and 4, direct calculations can bemade, as above.

Also in the dielectric arch-trapped evanescent resonant cavity(applicator), the field patterns of the TE₃₁₁ mode dominate. That modeshould not have any z-directed E component but the applicator mode has.This can be verified by microwave modelling, but the other components ofthe TM₃₁₁ mode (xy-plane H fields with maxima at the ceiling and floor,and xy-plane E fields with maxima at half height) are “hidden” since theTE₃₁₁ mode has those same components. In conclusion, the cavity mode isa hybrid HE₃₁₁ mode, where the cavity field intensities of the TE typeare stronger than those of the TM type.

The advantage of having an essentially constant resonance frequency willbe further discussed in the following.

It has been found by microwave modelling that the resonant frequency ofan applicator as above varies exceptionally little with even very largeload variations, such as from less than 1 mL in a small vial to over 50mL in a container as in FIGS. 2 and 3. The loads are then polar liquids,also with highly variable permittivities and loss factors. Frequencyvariation may then be as low as within 1 MHz.

The cavity disclosed in FIGS. 6 and 7 was modelled and the result fromthe modelling is presented in table 1 (below).

The load had a diameter of 9 mm and 15 mm high cylinder (no glass vial),positioned with its top about 2 mm below the cavity ceiling. The antennaprotrusion was quite small, in practice being in the same plane as thecavity wall (still with a hole in the ceramic block).

The ceramic permittivity was 7.5-j0.0125 throughout; this corresponds toa penetration depth of 4.2 m.

TABLE 1 Load Res. freq. Coupling Q0 value permittivity MHz factor(Prony) Remarks Empty 2471 0.22 O — — 10-j2 2466 0.17 O — Low-loss 25-j62467 0.13 O — Standard 78-j10 2465 0.16 U — Water at 20° C. 60-j2 24660.14 O — Water at 100° C. O = overcoupled; U = undercoupled.

Now different aspects of the microwave feeding means will be discussed.An interpretation of the function of the hybrid HE mode is that there isa balance between its TE and TM “parts” that changes with the loading.Dielectric loads that have a significant axial dimension typicallycouple more strongly to TM than TE modes and offsets the inherentlyhigher resonant frequency tendency of the TM mode part.

As a consequence of this interpretation, it becomes important to use afeeding means which does not inherently influence the balance betweenthe TE- and TM-type mode part relationships. Hence, if only the TE partis fed, the TM part can “freely” adapt to the variable load. Since theTM-type mode part lacks only one component—H_(z)—that becomes apreferred choice. This field component is strongest at the half heightof the circular periphery; there are maxima at 0°, 60° and 120°. Hence,a vertical slot feed at 0° or 120° is feasible. The complementary Efield to obtain a Poynting vector is then horizontal radial. The feedconfiguration is shown in FIG. 4; there is a normal TE₁₀ waveguidebeside the cavity, with a vertical slot at the end.

The envelope of the Hz field in a very similar scenario, at half thecavity height, is shown in FIG. 5. The field pattern 12 in thedielectric wall structure resulting from the TE₃₁ mode part isschematically illustrated.

Another possibility is to excite the “rotational” Hz field at 30° (whereit changes sign; there is no horizontal H field at half the cavityheight) by a coaxial probe, and then at the same time obtain fieldmatching to the horizontal radially inwards-going E field. That is shownin FIGS. 2 and 3.

Even if a desired function of reduced variation of the resonantfrequency with different loads in principle occurs with a thin and lowpermittivity dielectric insert into the applicator, a preferredembodiment is that the dielectric material used in the dielectric wallstructure (or cladding) should have such a high permittivity that asubstantial part of the oscillating energy is bound to the peripheryregion. The only presumption for a HE mode to exist is that thepermittivity (∈) is greater than 1. This results in a wide variety ofcombinations of the permittivity and the thickness of the dielectricwall structure. E.g. if ∈ is above 9, the (ceramic) cladding becomesrather thin, resulting in possible tolerance problems. For practicalreasons the permittivity is preferably between 4 and 12. Between 6 and 9seems to be the most desirable; the thickness is then between 8 and 6mm.

For completeness it should be noted that the thickness of the dielectricwall structure is not related to the standard theory for common trappedsurface waves which requires a thickness not greater that

$T = {\frac{\lambda_{0}}{2\sqrt{ɛ - 1}}.}$

One design consideration is that it may be more difficult to metallisethe outer surface of the ceramic than to leave an air distance betweenit and the cavity periphery. According to one embodiment of the presentinvention it has been found that a distance of 2 to 4 mm is feasible, incases where a minimum distance is desirable for achieving a very smallapplicator.

The so far described applicators have a small distance between the tileand the outer metal wall of the applicator; the reasons for this arethat a) metallization can then be avoided, and b) the mode field patternis not influenced much (i.e. the mode remains of the TEm;1 type (notwith higher second index than 1). This results in a conveniently smallapplicator. Applicators having a small distance between the peripherywall and the dielectric wall structure will be further described inconnection with FIGS. 6-10.

There are several advantages by increasing the distance between thedielectric structure wall and the periphery wall.

One advantage is that there is then no need to arrange a hole in thedielectric wall structure for the microwave feeding means. This in turnmakes it cheaper to manufacture the device.

Another advantage is that the near-field generated by the feeding meansbecomes more symmetrical.

These and other advantages will be further discussed in the followingwhere references are made to the FIGS. 14 and 15.

When the distance between the dielectric wall structure and theperiphery wall is increased to at least 15 mm, a second trapped surfacewave occurs in that region and the axial magnetic field of the modechanges sign in the dielectric wall structure.

The mode then becomes of the same kind as the basic (nowCartesian/rectangular) TM-zero dielectric-slab type. If the applicatoris circularly cylindrical, a number of standing (integer wavelengths)waves will occur circumferentilly, with the right dimensions.—Such anapplicator will still retain the radial index 1 inwards (where theload(s) is/are), but may be easier to feed if very large (exceeding 300mm or so at 2455 MHz, corresponding to circumferential index 10 or more(if 10, there are 20 standing wave maxima around the periphery). Aparticular advantage is that the feed needs not to be close to thetiles; near-field excitation resulting in risks of arcing or localoverheating of the tile are drastically reduced in high power systems.

It has turned out that it is possible to use a larger distance (25 mm ormore at 2450 MHz) between the inner surface of the periphery wall andthe dielectric wall structure. One may then obtain two different fieldtypes in the dielectric structure—it is to be note that the modereference is no longer to the whole cavity but instead only to thedielectric structure with wave energy propagating along in thecircumferential cavity direction (to set up the cavity mode), and inrectangular notation. The two mode types are then dominantly TM₀ andTM₁. In the former case, there is no polarity change across thedielectric structure, and in the latter case there is one.

It turns out that the resulting cavity mode will have a lower first (thecircumferential) index with the ceramic TM₀ field than with the ceramicTM₁ field, in spite of the radial index now being 2. That means that inthis preferred case, the radial inwards evanescence will be slower andthe mode behaviour also be less influenced by the load. The load islocated close to the inner surface of the dielectric wall structure.Another important advantage is that the feeding means (between thedielectric structure and the periphery wall) can now be such thatinsignificant near-fields exist on the inner surface of the dielectricstructure under conditions of normal high power transfer (i.e. impedancematching). In a preferred embodiment the feeding means is a commonquarterwave radially directed coaxial metal antenna.

Arranging the dielectric structure at significant radial distance fromthe cavity periphery wall allows dual antenna constructions with a phasedelay, resulting in an essentially unidirectional energy to flow insidethe cavity in the circumferential direction. Several types of suchantennas exist and can be used. Such antennas are typically easier todesign and become smaller with the ceramic TM₁ mode than with the TM₀mode, and since the circumferential mode index is higher in the formercase, the distance between the minima which will occur due toimperfections of the system becomes smaller, which is advantageous.

The radial airspace between the periphery wall and the dielectricstructure is up to half a free-space wavelength, which in a preferredembodiment is 20-30 mm. Either of the rectangular ceramic mode TM₀ orTM₁ is used, and TM₀ is typically preferred and is also what is obtainedwhen the distance between the periphery wall and the dielectricstructure is short.

Thus, FIGS. 14 and 15 illustrates two embodiments of microwave heatingdevices provided with large radial airspaces according to the presentinvention.

FIG. 14 is a cross-sectional view of a circular cylindrical cavityincluding a periphery wall 2, an airspace 18 between the periphery walland the dielectric wall structure 8 that encloses the load cavity 6. Afeeding means 10 is arranged through the periphery wall.

FIG. 15 shows a cross-sectional view of a sector-shaped microwaveheating device that in addition to the items of the embodiment in FIG.14 includes two sidewalls 4.

Since the operating resonance frequency is essentially constant, it maybe set to a suitable value in production trimming, by some means. It hasbeen found preferable to include a small radial metal post 22 (see FIG.2) positioned at the same location as the microwave feeding point but inthe next halfwave position of the field (which has two halfwaves in FIG.2 as drawn; that also applies to FIGS. 5 and 13). The metal postprovides an about 50 MHz downwards adjustment of the resonant frequencyin the 2450 MHz band, without any detrimental effects. The opening mayhave a diameter of 4 mm and the post is then less than 2 mm.

Since the hybrid mode is evanescent radially inwards, towards the “axistip”, there will be no or very weak fields there. In particular, sincemuch of the energy coupling to the load is via the horizontal H fieldsand these are zero at the half height, quite large non-disturbing andnon-radiating holes can be made in the radial cavity sides in thatregion.

A large load close to the “axis tip” will couple rather weakly (asdesired) and not change the resonant frequency much. However, a smallload in that same position may couple too weakly. If the very small loadposition is changed radially outwards along the dashed line 24 indicatedin FIG. 2, the coupling will become stronger and the heating efficiencywill increase. This allows an even larger latitude in load sizes anddielectric properties than with a fixed load position.

A practical simplification is to use flat tiles rather than a 120° (orso) curved one (as in FIGS. 3-5). It has been found that four such tilesas shown in the illustration in FIG. 2 is feasible. A smaller numberwill distort the delicate balance between the TE an TM mode parts of thehybrid mode in the cavity.

Microwave losses in the ceramic tiles cannot be avoided. As a matter offact these ultimately determine how small loads can be heatedefficiently. However, efficient heating of very small loads is difficultto control, due to the minute energy requirement. With “controlled”losses in the ceramic tiles, these can be said to be connected inelectric parallel with the load and thus limit the “voltage”. Thisresults in a maximum heating intensity in the load when it absorbs thesame power as the tiles (and also the cavity metal walls), and thisintensity then falling off rather than remaining constant if theabsorption capability of the load decreases further.

As expected, a typical system becomes overcoupled for small loads andundercoupled for large loads. The coupling can of course be changed sothat critical coupling (and thus maximum efficiency) occurs for asuitably specified load.—It is then possible to further employ thenon-linear properties of magnetrons, by choosing the mismatch phase (bythe length of the feeding waveguide) such that operation is in the(higher efficiency) sink region with a large load, and in the (lowefficiency but stable) thermal region for small loads. By such a design,the useful load range can be increased, and the risk of magnetron damagewith a small load or empty be drastically reduced (the base loading ofthe ceramic tiles and by the cavity wall losses also contribute to thelatter).

A second preferred embodiment of the present invention comprises a groupof different variants that all fulfil the following design goals:

-   1) to provide an inexpensive small applicator, e.g. for only 1.0 mL    liquid loads and the simplest possible system having no movable    parts.-   2) to facilitate dielectric property and self-heating testing of    ceramic tiles with minimum machining.

As for the first preferred embodiment the cavity carries a dominatingmode which is evanescent radially inwards towards the axis of a circularor sector-shaped cavity, in an airfilled region being either very smallor at least trapezoid (triangular is preferred), so that resonancesdetermined by the load itself and this workspace are deprecated.

There may be further ways of optimisation towards a still smallerresonant frequency difference for different load permittivities, by forexample deviations by “bulges” from the straight flat ceramic slabsides.

FIGS. 6-9 illustrates different variants of the second preferredembodiment. The triangular applicator, as in FIG. 7, is basically just adistorted sector-shaped design for resonance of the mainly HE typehybrid arch surface mode. It has been found that the flat instead ofarched ceramic does not give as good results with regard to frequencyconstancy for different loads, but results may be sufficient if loadgeometry or volume constraints are introduced.

By making the airspace trapezoids (see FIG. 8) by truncating thetriangular cavity with a third side wall 4′, the two resonancescoincide, which is not so favourable but this variant may be improved byincluding a second dielectric wall structure 8′ along the third sidewall that essentially stabilises the field. This results in a morecompact cavity.

There is a possibility to compensate for a non-arched ceramic tile insingle or multi-tile applicators, by making its cross section(horizontal, with the applicator axis considered vertical) withnon-parallel sides. For practical manufacturing reasons, one side shouldthen remain flat. This is shown in FIG. 9. The advantages are then thatthe behaviour becomes more like that with a truly arched tile (as shownin FIG. 2), i.e. better frequency constancy to variable loads.

The general geometry of the second preferred embodiment is that of acylinder with triangular cross-section, containing a dielectric wallstructure having a rectangular cross section the base side. The cavityfeed is by a small, central coaxial antenna. The adaptation of resonantfrequency to about 2455 MHz (in view of the not exactly known ceramicpermittivity) is by changing the overall height. For that reason, theoriginal height should be higher than anticipated for 2455 MHzresonance, so that it can more easily be changed.

The shape is shown in the FIGS. 6 and 7. The triangle above the ceramichas a base side of 80 mm and a height of 54 mm. The vertical cylinderheight for about 2455 MHz resonance is about 61 mm, but the originalheight should be made 80 mm. The ceramic block has the horizontal sides80 mm and 10 mm (=the thickness) and extends all their way in thevertical direction.

There is a 2 mm airgap 18 between the ceramic block and the parallelcavity wall behind. Hence, the cavity without ceramic consists of atriangular plus a rectangular part. The latter being 80×12 mmhorizontally.

At the half height there is a centred coaxial feed with a correspondinghole through the ceramic. The hole is 8 mm in diameter.

There is a metal tube 20 (=wavetrap) with inner Ø 13 mm above the load,and height at least about 9 mm. The load axis and tube axis nominalpositions are 32 mm from the applicator tip. Also illustrated in FIG. 6is a top wall 14 and a bottom wall 16 that together with the side walls4 and the dielectric wall structure make up the closed cavity. In FIGS.6-9 the feeding means 10 is a coaxial probe.

In FIG. 10 is shown a schematic and simplified set up of 6 microwaveheating devices as the one illustrated in FIG. 7 arranged together.Please observe that no feeding means are included in the figure.

In an exemplary embodiment the cavity being a cylinder having a circularcross-section and is provided with one single feeding means that createsa single standing wave pattern within the cavity. This embodiment isprimarily intended for heating multiple equal loads locatedsymmetrically as illustrated in the schematic drawing in FIG. 11 thatshows a cavity provided with 6 loads.

The standing wave pattern may be of the HE_(6,1) mode and have one loadat each field maximum, i.e. 12 loads, placed 30° apart or 6 loads (everysecond field maximum, i.e. 60° apart) or 4 loads (every third fieldmaximum, i.e. 90° apart) or 3 loads (i.e. 120° apart) or 2 loads (i.e.180° apart) or naturally one single load (schematically illustrated inFIG. 12).

FIG. 11 shows a circular microwave heating device with dielectric wallstructure 8 and a feeding means 10. The device may be in the HE_(3;1;1)mode and there will then be 6 field periods, so that 6 equal loads 6arranged in a circular fashion will be equally treated. Since the systemresonance Q factor can be made as high as desired (due to the modeevanescence), there can actually be an extremely similar “impinging”field to all loads. It is now possible to choose the load locations inrelation the positions of the standing magnetic and electric fields, sothat the loads are treated by equivalent current or voltage sources,respectively.

If the loads are not equal, the result may be a negative or positivefeedback of relative heating; for example by a hotter load of a numberof otherwise equal loads being heated less, or for example by a largerload being heated more strongly—or vice versa, which is of course notdesirable.

In a third preferred embodiment the cavity has a smaller size, and theperiphery wall and the dielectric structure have circular cross-sectionsconcentrically arranged with regard to each other. Naturally, thisembodiment also covers variants where the periphery wall and thedielectric structure have a cross-section that is a part of a circle.

In a specific example the outer radius of the dielectric structure 8 (inFIG. 13) with a permittivity of 9 is 50 mm (which also is the radius ofthe inner surface of the periphery wall) and an opening 6 for the loadwith a radius of 20 mm. FIG. 13 illustrates the field pattern 12 in asemicircular cavity provided with feeding means 10 working at 2450 MHzat the lowest part in the figure. The field pattern will then have twowhole and two half waves. As an alternative the centre angle may insteadbe 120° giving the same function. The height of the cavity is about 50mm (e.g. 49 mm).

In this embodiment where the radial thickness of the dielectric wallstructure (ceramic) is large and the arch-trapped evanescent resonanceprimarily takes place in the dielectric structure.

According to a fourth preferred embodiment of the present invention twohybrid modes, HE_(m2;2;p) and HE_(m1;1;p), with m2>m1, are used bothbeing resonant at the same frequency.

The coupling factor from a simple radial feeding antenna will becomedifferent for the two modes, since the fields of the HE_(m2;2;l) modeare more tightly bound to the dielectric and therefore couples lessstrongly then the HE_(m1;1;l) mode which has a more constant field nearthe cavity periphery wall.

A cavity with a large load will get a lower quality factor (Q value),since stationary conditions occur after fewer retro reflections in thecavity. Therefore, there will always be a tendency for the couplingfactor of a single mode cavity with a fixed antenna to go fromundercoupling (the coupling factor<1) towards overcoupling (the couplingfactor>1) when the load is reduced.

A design goal for a single mode resonant cavity for heating is thereforeto set the coupling factor not to be too low for the largest (or moststrongly absorbing) load, to be about 1 (critical coupling, resulting inimpedance matching and thus maximum system efficiency) for the mosttypical load requiring high power, and not to be too high for thesmallest (or weakly absorbing) load.

When two simultaneous modes are used to heat a load, one has to observethat these are almost always orthogonal. That means the power beingtransferred independently from the feed structure to the two modes, sothat the power absorption will come from independent modes. However,since the modes have a common feed, their relative amplitudes (and bythat their individual power transfer to the load) will depend on severalfactors such as the coupling impedances and feed to mode field matching.The resulting heating pattern will be a result of the vector summationof the two mode fields, since the situation is time-harmonic (the samesingle frequency is used).

Thus, according to the fourth embodiment the dynamic range of the systemis extended by using the HE_(m2;2;l) mode to heat small loads since itscoupling factor for such loads is smaller than that of the HE_(m1;1;l)mode—and by using the HE_(m1;1;l) mode to heat larger loads since itscoupling factor for such loads is larger than that of the HE_(m;2;l)mode. The HE_(m2;2;l) mode will be strongly undercoupled for large loadsand thus not disturb the action of the HE_(m1;1;l) mode. For small loadsthe HE_(m1;1;l) mode will be overcoupled and may then disturb the actionof the desired HE_(m2;2;l) mode in that case.

FIG. 16 illustrates a microwave heating device according to the fourthembodiment of the present invention. The device comprises asector-shaped cavity comprising a periphery wall 2 and two sidewalls 4″that encloses the dielectric wall structure 8″ and the load 6. Thedielectric wall structure has the form of two equal, flat tiles thatextend all the way from the bottom wall (not shown in FIG. 16) to thetop wall (not shown in FIG. 16) of the cavity. The tiles are typically10 mm thick, 80 mm high and have typically an ∈ value of 8, the radiusof the cavity is 85 mm and the sector angle is 120°.

One important feature of the fourth embodiment is that there is asignificant radial distance between the curved periphery wall 2 and thedielectric wall structure 8″ where air spaces 18′ are formed. This isimportant since only then can two close resonant frequencies for modesof the HE_(m1;1;p) and HE_(m2;2;p) types easily be found and used.

As mentioned in relation with the embodiment shown in FIG. 2 a metalpost (not shown in FIG. 16) may be used for fine-tuning of the resonantfrequency of the HE_(m1;1;p) mode. There may also be a need to fine-tuneto zero difference between that resonance and that of the HE_(m2;2;p)mode. This is achieved by moving the tiles inwards in the radialdirection.

Also shown in FIG. 16 is a microwave feeding means 10, here in the formof a coaxial antenna. The insertion depth of the antenna is sensitivefor the proper function of the microwave device. In the case illustratedin FIG. 16 the antenna insertion depth into the cavity is about 7 mm andits diameter is about 3 mm.

The frequency of both resonances is reduced somewhat with increasedinsertion depth—which of course also results in an increase of thecoupling factor. In the shown illustration the load may have diametersranging from 3 mm to 20 mm, and heights from 20 to 60 mm.

A number of data modelling of the system according to the fourthembodiment have been performed primary to investigate the frequencybehaviour for different loads. This investigation confirms that a highefficiency is maintained under all conditions, with regard to theresonant frequency variability.

Thus, the dual hybrid arch surface mode cavity according to the fourthembodiment of the present invention provides a high heating efficiencyfor an exceptionally wide range of loads. The reason is that, with thesame unchanged feeding means, the modes are interchangeably over- andundercoupled for large and small loads. This results in at least one ofthem couples well to almost any reasonable cavity load. This extends therange of use to also small loads of about 0.1 mL (depending on thepermittivity and how much overpowering is to be used). Such overpowering(perhaps up to 700 W input power) may be used with such small loads,since the cavity antenna is not located close to any ceramic tile whichwould otherwise cause field concentrations.

It has also turned out that the field pattern in the dual hybrid archsurface mode cavity has an improved coupling to some types of very smallload geometries, in comparison with a single hybrid mode cavity.

The dual hybrid arch surface made cavity also provides possibilities fora quite even heating pattern in several load geometries—both large andsmall, and not necessarily in the shape of a vial. Examples of suchextended use is heating of thin and horizontally flat loads, and use ofa flow-through load application for processing of solid, semisolid orliquid loads in a type having a diameter up to 40 mm.

Finally, FIG. 17 shows a block diagram of a system for using themicrowave heating device according to the present invention. An operatorcontrols the system via a user interface (not shown) connected to acontrol means that inter alia controls the microwave generator withregard to e.g. the frequency and energy. The microwave generator appliesthe microwaves to microwave heating device via the microwave feedingmeans. The control means may also by provided with measurement inputsignals from the microwave heating device; these signals may represente.g. the temperature and pressure of the load.

The present invention also relates to a method of heating loads in amicrowave heating device or in a microwave heating system according toany above-mentioned embodiment. The method comprises the steps ofarranging a load in the cavity and applying microwave energy at apredetermined frequency to the microwave heating device in order to heatthe load(s).

Furthermore, the present also relates to the use of a microwave heatingdevice or a microwave heating system according to any above-mentionedembodiment for chemical reactions and especially for organic chemicalsynthesis reactions, and also the use of the above method for chemicalreactions and especially for organic chemical synthesis reactions.

The present invention is not limited to the above-described preferredembodiments. Various alternatives, modifications and equivalents may beused. Therefore, the above embodiments should not be taken as limitingthe scope of the invention, which is defined by the appending claims.

1. A microwave heating device for at least one heating load, themicrowave heating device comprising: a cylinder-shaped cavity enclosedby a periphery wall, said cavity including a microwave feeding device;and a dielectric wall structure arranged inside said cavity between saidperiphery wall and said at least one load, wherein said microwavefeeding device is arranged to generate a microwave field being an archsurface hybrid mode having TE and TM type properties inside said cavityto heat the at least one load, wherein the feeding device includes aradially directed coaxial antenna and the microwave field is fed to thefeeding device using an Hz component of the microwave field, wherein athickness of the dielectric wall structure is dimensioned to balancebetween components of the hybrid mode in relation to a permittivity ofthe dielectric wall structure such that a substantial part ofoscillating energy of the microwave filed is bound to a periphery regionof the dielectric wall structure.
 2. The microwave heating deviceaccording to claim 1, wherein said dielectric wall structure is incontact with an inner surface of the periphery wall.
 3. The microwaveheating device according to claim 1, wherein said dielectric wallstructure covers the whole inner surface of the periphery wall.
 4. Themicrowave heating device according to claim 1, wherein said dielectricwall structure is arranged a distance from the inner surface of theperiphery wall.
 5. The microwave heating device according to claim 1,wherein said dielectric wall structure comprises a number of tiles thatessentially follow the shape of the periphery wall.
 6. The microwaveheating device according to claim 1, wherein said cavity comprises anupper wall and a lower wall.
 7. The microwave heating device accordingto claim 1, wherein a metal post is arranged in an opening of theperiphery wall for adjusting the resonant frequency.
 8. The microwaveheating device according to claim 1, wherein the load is adapted to beplaced close to the centre of the cylinder-shaped cavity.
 9. Themicrowave heating device according to claim 1, wherein the feedingdevice is a coaxial feeding device.
 10. The microwave heating deviceaccording to claim 1, wherein for the hybrid mode the circumferentialinteger index m is less than 4, the radial index n=1 and the axial indexp being an integer>0.
 11. The microwave heating device according toclaim 1, wherein said cavity has a circular cross section.
 12. Amicrowave heating device for heating at least one load, the microwaveheating device comprising: a cylinder-shaped cavity having a peripherywall and two sidewalls attached to said periphery wall and to each otherwith an intermediate angle being less than 360°, the cavity having amicrowave feeding device; and a dielectric wall structure arrangedinside said cavity between said periphery wall and said at least oneload, wherein said microwave feeding device is arranged to generate amicrowave field being an arch surface hybrid mode having TE and TM typeproperties inside said cavity to heat the at least one load, wherein thefeeding device is one of a slot along one of the two sidewalls and aradially directed coaxial antenna, and the microwave field is fed to thefeeding device using an Hz component of the microwave field, wherein athickness of the dielectric wall structure is dimensioned to balancebetween components of the hybrid mode in relation to a permittivity ofthe dielectric wall structure such that a substantial part ofoscillating energy of the microwave filed is bound to a periphery regionof the dielectric wall structure.
 13. The microwave heating deviceaccording to claim 12, wherein said intermediate angle is 120°.
 14. Themicrowave heating device according to claim 12, wherein saidintermediate angle is 60°.
 15. The microwave heating device according toclaim 12, wherein said intermediate angle is 180°.
 16. The microwaveheating device according to claim 12, wherein said periphery wall has acurved shape.
 17. The microwave heating device according to claim 12,wherein said periphery wall is a plane.
 18. The microwave heating deviceaccording to claim 12, wherein for the hybrid mode the number of halfwaves inside the cavity is 1 or 2, the radial index n=1 or n=2 and theaxial index p=1.
 19. The microwave heating device according to claim 12,wherein said cavity has a cross section being a sector of a circle. 20.The microwave heating device according to claim 12, wherein saidperiphery wall has a cross section being a sector of a circle and thatsaid dielectric wall structure being two equal, flat tiles, wherein twoarch surface hybrid modes, HE_(m2;2;p) and HEmi_(;1;p) with m2<m1, aregenerated in said cavity, both hybrid modes being resonant at the samefrequency.
 21. The microwave heating device according to claim 20,wherein air spaces are formed between the flat tiles and the peripherywall.
 22. A microwave heating system, comprising a plurality of themicrowave heating device according to claim 12, wherein the plurality ofmicrowave heating devices allow parallel handling and heating of loads.23. The microwave heating device according to claim 12, furtherincluding a resonant applicator, the resonant applicator having a firstindex of 2 to
 10. 24. The microwave heating device according to claim12, wherein the thickness of the dielectric wall structure is about 7mm.
 25. The microwave heating device according to claim 12, wherein thepermittivity of the dielectric wall structure is about 7.5.
 26. A methodof heating at least one load in a microwave heating device comprising:providing at least one microwave heating device according to claim 12;arranging the load in the cavity of the at least one microwave heatingdevice; and applying microwave energy at a predetermined frequency tothe microwave heating device to heat the at least one load.